As a four-wheel drive vehicle that drives the front and rear wheels, there is a vehicle that varies the torque-distribution ratio of the front and rear wheels according to the driving conditions. This kind of four-wheel drive vehicle comprises: main drive wheels, which are on one of the front wheel side and the rear wheel side, to which the drive torque is directly transmitted from the transmission system; and auxiliary drive wheels, which are on the other one of the front wheel side and the rear wheel side, to which the drive torque is transmitted from the transmission system via a friction clutch mechanism, and controls the torque-distribution ratio of the front and rear wheels by adjusting the fastening strength of the friction clutch mechanism (for example, refer to Japanese patent application publication No. H10-6798 and Japanese patent application publication No. 2003-11685).
When a vehicle is turning, rotation difference occurs between the front and rear wheels, so in the case of a four-wheel drive vehicle in which the drivelines for the main drive wheel side and the auxiliary drive wheel side are connected, it becomes necessary to avoid the so-called tight-corner braking phenomenon by separating the drivelines for the main drive wheel side and auxiliary drive wheel side, and absorbing this rotation difference. Even though the rotation difference between the front and rear wheels is absorbed in this way, in order to avoid a drop in driving performance or a sudden change in torque, it becomes essential to gradually reduce the fastening force of the friction clutch mechanism and control the friction clutch mechanism in the slip state.
However, when attempting to control the friction clutch mechanism in the slip state, microscopic fastening and release of the friction plate is repeated, and there is a possibility that the so-called stick-slip phenomenon may occur. This kind of stick-slip phenomenon becomes the cause of vibration of the driveline on the auxiliary drive wheel side that has a small inertia force, and in a condition where the transmission state of the drive torque in the transmission becomes neutral, not only does this phenomenon cause vibration in the driveline of the auxiliary drive wheel side, but also causes vibration of the driveline of the main drive wheel side. When the driveline of the main drive wheel side vibrates in this way, it causes the transmission system that comprises a plurality of gears and the like to vibrate, as well as causes abnormal noise to be generated from the transmission system, so it is desired that vibration of the driveline of the main drive wheel side be suppressed.